Summary of Techniques: Problems attributable to electromagnetic inference (EMI) have increased with the recent functional progress and spread of electric and electronic devices. Electromagnetic noise is classified roughly into conducted noise and radiated noise. Methods of preventing problems due to conducted noises filter conducted noise by a noise filter. Methods of preventing problems due to radiated noise use a metal case to shield a space electromagnetically, place a metal sheet between wiring board, or coat the wires of cables with a metal foil. Although effective in electromagnetically shielding circuits and power blocks, these methods are unsuitable for shielding electromagnetic radiation generated by the screens of displays, such as CRTs and PDP, because those methods use opaque means.
The PDP is an assembly of a glass panel provided with data electrodes and a fluorescent layer, and a glass panel provided with transparent electrodes. The PDP generates a large amount of electromagnetic radiation, a large amount of near-infrared radiation and a large amount of heat when operated. Usually, a front panel is disposed in front of the PDP to shield electromagnetic radiation. The front panel must have a shielding function of 30 dB or above in the range of 30 MHz to 1 GHz to shield electromagnetic radiation emitted from the screen of the display. Infrared radiation of wavelengths in the range of 800 to 1,100 nm radiated from the screen of the display must be shielded because infrared radiation makes other devices, such as a remote controller for the remote control of VTRs and air conditioners, malfunction. The electromagnetic shielding metal mesh (lines) of the electromagnetic shielding sheet should not deteriorate visibility and the electromagnetic shielding sheet must have proper transparency (visible light transmitting property, visible light transmittance) to make images displayed by the display satisfactorily visible.
Since the PDP is provided with a large screen, such as a 37 in. screen, a 42 in. screen or a larger one, the electromagnetic shielding sheet used in combination with the PDP has thousands of horizontal and vertical lines defining openings. Since parts of an adhesive layer are exposed in the openings of the electromagnetic shielding sheet, the adhesive layer exposed to the adverse effects of an etchant during etching is colored, and the adhesive layer is deteriorated by an alkaline resist remover for removing a resist film after etching.
Prior Art: To ensure improved visibility of displayed images, the front panel is required to have a uniform, stable mesh structure capable of electromagnetic shielding, having proper transparency (visible light transmittance), not colored in an undesired color and not subject to peeling.
A front panel having a mesh structure disclosed in JP 5-283889 A has the following structure: (base)/(transparent anchor layer)/(electromagnetic shielding layer). The electromagnetic shielding layer has a meshed pattern and is formed by an electroless plating process. A method of forming a metal mesh for an electromagnetic shielding sheet disclosed in JP 09-293989 A uses a photoresist process. An electromagnetic shielding structure disclosed in JP 10-335885 A is formed by laminating a plastic film provided with a copper foil formed in a geometrical pattern by photolithography to a plastic plate. Although those prior art electromagnetic shielding sheets formed by the prior art methods have a mesh structure, only the metal mesh is coated with an antirust layer, and the antirust layer is not formed over the entire base and the adhesive layer. The prior art methods mention nothing about forming the antirust layer over the entire base and the adhesive layer. Generally, the electromagnetic shielding sheet is disposed with the base facing the viewing side. Therefore, it is difficult to connect a grounding electrode of the electromagnetic shielding sheet to a ground.
An electromagnetic shielding sheet disclosed in JP 11-298185 having a structure (transparent base)/(black layer)/(patterned metal layer)/(antirust layer) is formed by a patterning method of forming a pattern (a meshed pattern, as in the present invention) that superposes a black layer and a patterned metal layer on a patterned resist film, washes off the black layer and the patterned metal layer together with the resist film, and then forms the antirust layer. However, the antirust layer cannot be formed between the transparent base and the black layer or the metal layer, or in openings. Furthermore the minute meshes cannot be precisely formed because the pattern (the meshed pattern) is not formed by an etching process. Parts of an adhesive layer or the base exposed in the openings to form transparent parts are colored or yellowed by the agency of an etchant used in the corroding process. Such colored parts affect the color tone of displayed images, and the coloring of those parts reduces the adhesive strength of the adhesive layer. In some cases, metal ions, such as Fe3+ ions, contained in the etchant and penetrated into the adhesive layer and the base deteriorate coloring matters added to the adhesive for color correction and infrared ray absorption. When an electromagnetic shielding sheet is bonded to a transparent base or the screen of a display with an adhesive, bubbles B remain in the openings in the mesh structure as shown in FIG. 7, and light is scattered at the interface between the bubbles and the adhesive to increase haze value. A filter disclosed in JP 2000-227515 A is formed by laminating a near-infrared absorbing filter to the foregoing electromagnetic shielding sheet, and has both an electromagnetic shielding ability and a near-infrared absorbing property.
An electromagnetic shielding sheet fabricating method of fabricating this prior art filter needs an additional process for bonding the near-infrared absorbing filter to the electromagnetic shielding sheet with an adhesive.